1. Field of the Invention
The present invention relates to an information communication system for data transmission/reception between a host computer and a plurality of terminals connected to the host computer via lines. More particularly, the present invention relates to an information communication system capable of performing both point-to-point and point-to-multipoint communications using same lines.
2. Description of the Related Art
There is known an information communication system which supports a point-to-point (PTP) communication for the transmission of data from a host computer to a particular terminal and a point-to-multipoint (PTMP) communication (inclusive of broadcast, and limited broadcast) for the transmission of same data from the host computer to a plurality of terminals, as disclosed for example in JP-A-2-14362G. With this information communication system, in transmitting data from a host computer to a particular terminal, the host computer instructs an interface unit to connect a line to the particularly terminal and thereafter sends the data from a data transmission/reception unit to the terminal via the connected line. In transmitting same data from the host computer to a plurality of terminals, the same procedure as the data transmission to a particular terminal is repeated as many times as the number of terminals. If lines including a particular line have been connected already, the data transmission/reception unit checks information added to the data so as to transmit it only to the particular terminal. In an alternative case, the host computer is connected to all the terminals via lines in advance so that the host computer can transmit data to desired terminals, by providing additional information to the data to be transmitted.
With this conventional technique, however, the host computer is required to be connected to a destination terminal each time data is to be transmitted, if it is not still connected to the line. There arises, therefore, the problem that data transmission is delayed by the time required for the interface unit to connect the line to the host computer. On the other hand, in the case where the line to a destination terminal has been connected already to the host computer (or if all lines are being connected to the host computer in advance), the amount of additional information representing destination terminals increases as the number of destination terminals increases (the additional information amount becomes great particularly when same data is transmitted to all the terminals), resulting in an increase of the amount of total information to be transmitted. An increase of the additional information increases the load to be processed by the host computer. There arises, therefore, the problem that a PTMP communication for transmitting same data to a plurality of terminals cannot be performed efficiently even if all lines are being connected to the host computer in advance.
For a conventional duplex communication system, there is known a technique of ensuring the integrity of communication data when the system is interchanged from an in-operation (master) system to a stand-by (slave) system, as described for example in JP-A-63-61339. According to this technique, the in-operation system is manually changed to the stand-by system after data to be inherited (received data, transmitted data, destination address waiting for data transmission, and the like) has been transferred from the in-operation system to the stand-by system. In this manner, the integrity of communication data has been retained. However, information to be inherited is not always possible to be transferred to the stand-by system from the in-operation system in trouble, posing a problem of disability of switching between duplexed systems. Also, although this conventional technique considers the integrity of communication data transmitted from the host computer to a terminal, it does not ensures the integrity of communication data transmitted from the terminal to the host: computer. There arises, therefore, the problem of missing communication data transmitted from a terminal when one system of the duplexed host computer becomes defective, although the host computer can still continue its operation.
As another conventional technique, there is known a system which provides various information services via communication lines, as presented by the paper, p-142, in The Institute of Electronics, Information, and Communication Engineers, 1992, Spring Meeting. In this system, the host computer has redundant internal elements to enhance the resistance against failure and ensure the data integrity when any redundant internal element becomes defective. To the memory space of this host computer ensuring a high operability because of redundancy, a communication data FIFO (First-in and First-out) queue and a status information storage area are assigned. The status information includes the status of reading data from the FIFO queue, and the status of writing data into a queue of a line control unit from which communication data is transmitted to a line. The status information is updated each time the communication process is executed. When the in-operation system of redundant elements on the data communication path becomes defective, the communication data is inherited in accordance with the status information, to avoid missing data and duplicated data. However, this conventional technique does not disclose how the host computer processes received data, although it describes how to transmit data from the host computer. This technique does not concern about the case of a plurality of destination terminals.